1. Field of the Invention
The present invention relates to low-pass filters and, in particular, to a low-pass filter that includes an internal coil.
2. Description of the Related Art
An example of a known low-pass filter is a multilayered low-pass filter described in Japanese Unexamined Patent Application Publication No. 2008-187418. The multilayered low-pass filter described in this patent document includes a multilayered substrate, a first coil, a second coil, and a capacitor. The multilayered substrate includes a plurality of dielectric layers stacked on each other. Each of the first coil and the second coil has a substantially spiral shape formed by a plurality of conductive layers on dielectric layers, the conductive layers being connected to each other through a via-hole conductor extending through the dielectric layer disposed therebetween. The first coil, the second coil, and capacitors are incorporated in the multilayered substrate and form a T-type low-pass filter.
The multilayered low-pass filter described in the above-mentioned patent document needs a lower frequency of the pass band. One approach to making the frequency of the pass band low can be an increase in the inductance of each of the first and second coils. To this end, the number of the conductive layers of each of the first and second coils can be increased.
However, for the low-pass filter described in the above-mentioned patent document, if the inductance of each of the first and second coils is increased, a problem arises in that the Q value decreases. In more detail, one of the conductive layers of each of the first and second coils is drawn to the side surface of the multilayered substrate, and thus each of the first and second coils is connected to an external electrode disposed on the side surface of the multilayered substrate. The external electrode is in contact with a land of the circuit substrate at its bottom when the multilayered low-pass filter is mounted on the circuit substrate. A high-frequency signal input from the circuit substrate to the multilayered low-pass filter travels through the bottom of the external electrode upwardly in the stacking direction and is input to the first coil through the region in which the conductive layer and the external electrode are connected. The high-frequency signal passing through the first and second coils travels through the region in which the conductive layer and the external electrode are connected downwardly in the stacking direction and is input to the circuit substrate through the bottom of the external electrode.
For the above-described multilayered low-pass filter, if the number of the conductive layers of each of the first and second coils is increased and the distance between the bottom of the external electrode and the region in which the conductive layer and the external electrode are connected is increased, the value of resistance of the multilayered low-pass filter is increased. As a result, the Q value of the multilayered low-pass filter is reduced.